The nuclear receptor CAR (NR1I3) is responsible for the increase in hepatic drug metabolism that results from exposure to diverse xenobiotics. These include the well known inducer phenobarbital, which functions as an indirect CAR activator and a large number of species-specific agonist or inverse agonist ligands. CAR activation also induces an acute, but strictly limited proliferative response, and blocks apoptosis. In accord with this, phenobarbital is a well studied liver non-genotoxic carcinogen and tumor promoter, and we demonstrated that CAR mediates these effects. The ability of CAR to drive liver tumorigenesis suggests, by analogy with hormone dependent estrogen and androgen receptor driven breast and prostate cancers, that antagonizing CAR could have beneficial effects in cancer treatment and chemoprevention. Our preliminary results strongly support this: the mouse CAR inverse agonist androstanol can inhibit hepatocyte proliferation and tumor growth, and may actually shrink established tumors. Additional preliminary results demonstrate a powerful functional synergy between CAR and -catenin that results in both unlimited hepatocyte growth and hepatocarcinogenesis. The current proposal addresses two specific hypotheses: 1) CAR and -catenin function synergistically to drive hepatocyte proliferation and tumorigenesis. 2) Reversing CAR transcriptional activation by inverse agonists exerts anti-proliferative effects in normal hepatocytes and liver tumors, and will have chemopreventative effects in hepatocarcinogenesis. Three specific aims address these hypotheses: 1. Characterize the molecular basis for the proliferative and anti-proliferative effects of CAR agonists and inverse agonists in cell culture and in vivo. 2. Define the mechanisms for the synergistic impact of combined CAR and -catenin activation in acute liver growth and chronic hepatocarcinogenesis. 3. Critically test the ability of CAR inverse agonists to treat and prevent liver cancer. These studies will provide important new insights into the molecular mechanisms that lead to liver cancer. More importantly, they will identify specific therapeutic strategies that will have direct relevance to human health.